This invention relates to a method and apparatus for manufacturing plastic bags by transversely cutting and sealing a plastic web at spaced locations, and more particularly to a method and apparatus for improving the efficiency at the stacking station of the bag making machine.
Bag making machines of various types are known, as generally exemplified in U.S. Pat. Nos. 5,338,281, 5,094,656 and 4,699,607, which are incorporated herein by reference. As disclosed in these patents certain types of plastic bags are typically manufactured by drawing a tubular plastic web from a supply roll, then punching holes to be inward of one edge of the film, then cutting and sealing the web transversely at a cut and seal station to form bags of selected lengths. The formed bags are then carried by a wicketer vacuum arm assembly to a stacking station where the bags are stacked onto a set of wicketing pins mounted to a wicketing stand which is carried by an intermittently moving conveyor. It is intended that the pre-punched holes in the bags align with the wicketing pins as the bags are delivered to the pins which then extend through the holes. Such alignment does not always occur, leading to damaged bags and jamming of the machine, which is the subject of the present invention. When a desired amount of bags are stacked the full wicketing stand is moved by the conveyor away from that stacking station, and a new wicketing stand including an empty set of wicketing pins moves into position to receive a new supply of formed bags. This procedure is repeated as each wicketing stand is filled and moved away.
Typical prior art bag making machines include various arrangements of the components including a supply roll and unwind station, a synchronizing station, a punch and registration station, a draw-to-length station, a cutting and sealing station, a transfer station and a stacking and delivery station. In such bag making machines the supply roll is either driven by a motor designated driven unwind or pulled by a capstan roller servo motor.
In a continuous feed unwind machine the web""s continuous motion is converted into a stop-and-go motion at the synchronizing station. In the xe2x80x9cgoxe2x80x9d portion of the stop-and-go motion the web is pulled by a set of draw rollers driven by a servo motor or by a clutch apparatus to the desired length (draw-to-length section). In the xe2x80x9cstopxe2x80x9d portion of the stop-and-go motion the cutting and sealing operations are performed. Prior to the cutting and sealing station one or more holes are punched through the web usually near its edge. These holes need to be at a correct distance from the web edge because downstream they must align with and descend onto wicketing pins extending from a wicketing stand carried by an intermittently moving conveyor.
A persistent and inherent problem in such bag making machinery is that as the fast moving web progresses in the web feed direction to the punch station, it periodically shifts, drifts or wanders transversely of said web feed direction. Holes in the web are then punched too close or two far from the edge of the web, and downstream those holes fail to properly align with the wicketing pins. As noted above, the consequences of such misalignment include damaged bags and jamming and interruption of the machine operation.
A technique in the prior art which attempted to solve the above-described problem included provision at the hole punch station of a punch carriage carrying the punch apparatus that is movable transversely of the web feed direction, and provision at the stacking station of a conveyor carriage carrying the wicketing pins that is similarly movable transversely of the web feed direction. The strategy was to monitor with a web sensor the transverse web shift in the vicinity of the hole punch station, determine the amount of this transverse shift and communicate this information to a controller, such as a PLC, which directed a servo motor and gear drive to move the conveyor carriage on which the wicketing pins were mounted, such that the pins would be re-positioned to better align with the holes made in the bags at the punch station. More specifically, the web sensor provided a directional signal which was converted by the controller to a directional output to the conveyor carriage servo motor and its gear drive. This prior art system further included a flexible drive cable connected from the conveyor carriage servo motor back to a gear drive at the hole punch carriage. For each full or partial rotation of this conveyor carriage servo motor and gear drive the flexible drive cable was intended to produce the same rotation in a similar gear drive coupled to the punch carriage, and to produce the same transverse movement in the punch carriage as was occurring with the conveyor carriage.
This prior art arrangement with a flexible drive cable has been found to have various serious drawbacks. At times this cable becomes bent or tangled, causing the hole-punching carriage not to follow the transverse movement of the conveyor carriage, with a continuation of the original problem, namely that plastic bags have holes punched in locations which will not properly align with the wicketing pins at the stacking station.
Another problem with this prior art flexible drive cable occurs when it becomes necessary to intentionally change the location of the hole punching apparatus, to allow the punched holes to be closer to or further from the web edge. In this prior art system this change is accomplished by manually uncoupling the flexible drive cable from the stacking station motor, manually rotating this cable clockwise if the punched holes need to be closer to the edge or rotating the cable manually counterclockwise if the punched holes need to be farther away from the web edge. Machine operators have to estimate how many manual turns of the cable will be needed to position the hole punch carriage in the desired location. After the flexible cable is re-attached manually, the bag-making machine has to be started and then run to produce enough bags to check if the punched holes in the web core at the desired distance. If the adjustment is unsatisfactory this whole sequence of steps has to be repeated multiple times.
A principal object of this invention is to overcome the problem of bags having punched holes which are not properly aligned with stacking pins at the stacking station. Accordingly, it is an object to better control the positioning of the hole punch carriage such that the holes are punched at the desired distance from the web edge. An additional object is to eliminate the flexible drive cable apparatus of the prior art used in an attempt to coordinate the punch station with the stacking station. A still further object is to provide a system for adjusting and controlling the transverse positions of both the hole punch and conveyor carriages instead of merely having the punch carriage attempt to follow the conveyor carriage.
To achieve these objects the invention provides a sensor/transducer and a servo motor for each of the conveyor and punch carriages, along with an appropriate servo controller and a web sensor near the punch station. If the web shifts transversely of the web feed direction, the web sensor outputs a directional signal to the servo controller which directs a servo motor coupled to the conveyor carriage to move similarly transversely. The sensor/transducer coupled to the conveyor carriage registers this movement of the conveyor carriage relative to a reference point and outputs a directional signal to the servo controller which directs the servo motor coupled to the punch carriage to move transversely as the conveyor carriage moved. The sensor/transducer at the punch station detects this movement of the punch carriage relative to a reference point and outputs a directional signal to the servo controller to be compared with the output signal from the conveyor carriage sensor/transducer. The servo controller continues to receive feedback from the two sensor/transducers and to output commands to the two servo motors. When the output signals from both sensor/transducers are the same, alignment and equilibrium will be achieved and the servo controller will cease directional outputs to the two servo motors
The invention thus includes a new bag making machine and method as described herein and a subsystem of a punch station and a stacking station with their respective sensor/transducers and servo motors coupled through a servo controller to achieve alignment of the punched holes in cut bags with wicketing pins at the stacking station.